How the imagery downloading loop is driven initially?

I am reading cesiumjs`s source code. I was confused by that when the camera position changed, how and from where cesium is to info the imagery provider to download new images. Can anyone help me?

Welcome to the Cesium Community Forum!

When the camera position changes, the engine uses a metadata file (ex. tilemapresource.xml for TMS) that contains the tile structure to figure out what tiles to load. You can adjust the level of detail of the imagery with the screen space error parameter.

What kind of project are you working on?